This invention relates to rechargeable or secondary batteries and, in particular, to rechargeable or secondary batteries using zinc negative electrodes.
Various types of zinc secondary batteries are known in the art. Typical batteries are Ni--Zn, Ag--Zn, Zn--O.sub.2 and MnO.sub.2 --Zn.
Zinc secondary batteries employ zinc negative electrodes which exhibit a decay or reduction in capacity with the repetitive charge-discharge cycling of the battery. As a zinc secondary battery and, therefore, each of its zinc negative electrodes, is cycled, the zinc active material of each electrode becomes redistributed over the surface of the electrode. This redistribution of active material or so-called "shape change" of each zinc electrode is a result of the solubility of the zinc active material in the alkaline electrolyte of the battery and causes a reduction in the capacity of the battery.
The solubility of the zinc active material in the battery electrolyte also causes an increase in the battery gas pressure due to the decreased availability of the zinc active material to recombine with oxygen generated during charging. It similarly leads to an increase in the rate of hydrogen evolution by the battery. Accordingly, a rapid buildup of gas pressure occurs in the battery, often requiring that the battery be vented. Constant venting of the battery, however, causes loss of water through electrolysis. This, in turn, causes dryout of the battery electrodes, reducing battery life.
Over the years, many different zinc negative electrodes have been proposed to reduce electrode shape change. One such zinc negative electrode is disclosed in U.S. Pat. No. 3,516,862 issued to W. Van der Grinten. In the '862 Patent, Ca(OH).sub.2 is added to the zinc active material (ZnO) to reduce the solubility of the active material through the formation of CaZn.sub.2 (OH).sub.6 (calcium zincate). In U.S. Pat. No. 3,816,178, Maki, et al. disclose a zinc electrode containing Ca(OH).sub.2 along with addition of oxides of lead. In U.S. Pat. No. 4,418,130, Soltis, et al. disclose the use of Ba(OH).sub.2 octahydrate. However, in these electrodes, the formed calcium or barium zincates experience thermodynamic instability due to dissociation in the battery electrolyte and a finite solubility as evidenced by improved but still low cycle life.
Also in the above-mentioned '614 application a zinc negative electrode is disclosed comprised of a zinc active material, a Ca(OH).sub.2 material and a conductive matrix containing a metal oxide material which is more electropositive than the zinc active material. The zinc electrode is utilized with an electrolyte having a low concentration of electrolyte constituent and the electrode, electrolyte and a positive electrode are arranged in a container to form a zinc secondary battery.
The Ca(OH).sub.2 material of the zinc negative electrode of the '614 application is in the range of 15-40 percent of the weight of the electrode and the metallic oxide material is in the range of 5-20 percent of the weight of the electrode. Also, the electrolyte contains an electrolyte constituent which is in the range of 5-20 percent of the electrolyte.
Also disclosed in the '614 application, in order to promote gas recombination, the zinc negative electrode is further configured as a split electrode with adjacent like electrode assemblies spaced by a porous hydrophobic element. Each electrode assembly includes an active element comprised of zinc active material, Ca(OH).sub.2 and a conductive matrix formed with a metallic oxide. One side of the active element abuts a metallic current collector element which, in turn, abuts a gas recombination catalytic element formed from a material more electropositive than zinc and having a chemical or electrochemical affinity for reacting with oxygen. The gas recombination catalytic elements of the electrode assemblies abut the hydrophobic element. The latter element and the electrode assemblies are enveloped by a separator material to complete the electrode.
While the zinc negative electrode of the '614 application provides improved stability, insolubility and gas recombination, there is still a need to provide the zinc negative electrode with added resistance to solubility.
It is, therefore, an object of the present invention to provide an improved zinc negative electrode and zinc secondary battery.
It is a further object of the present invention to provide a zinc negative electrode and zinc secondary battery with reduced shape change and solubility of the zinc electrode.
It is yet a further object of the present invention to provide a zinc negative electrode and zinc secondary battery meeting the above objectives and having improved gas recombination, permitting the battery to be sealed and maintenance free.